patch 8.2.2415: no way to check for the cmdwin feature

Problem:    No way to check for the cmdwin feature, cmdline_hist is now always
            enabled.
Solution:   Add has('cmdwin') support. Skip arglist test on Windows
            temporarily.

src/testdir/test_window_cmd.vim

@@ -19,6 +19,8 @@ func Test_window_cmd_ls0_with_split()
 endfunc
 
 func Test_window_cmd_cmdwin_with_vsp()
+  CheckFeature cmdwin
+
   let efmt = 'Expected 0 but got %d (in ls=%d, %s window)'
   for v in range(0, 2)
     exec "set ls=" . v